Projection welding is a known technique for permitting a weld-type securement between two thin overlapping metal sheets, particularly ferrous (i.e. steel) sheets. In projection welding, which is desirable because it permits use of conventional low-frequency (i.e. 60 cycle) voltage and current, one of the steel sheets is provided with a small projection extending transversely therefrom, with the tip of the projection being maintained in contact with the other overlapping sheet. The electrode of a welding gun is pressed into contact with the other steel sheet, generally in alignment with the projection, and a combination of force and electric current is applied to the electrode to effect welding together of the two sheets at the contact area defined by the projection, and to collapse the projection to thereby effect a weld nugget for securing the overlapping steel sheets together.
In the known projection welding technique, during a single projection welding operation, it has been conventional to apply alternating electrical current to the contact electrode over a fairly long time period, whereby several cycles of AC current are applied to the electrode to permit performance of a single projection weld. This long current application time, and the difficulty in achieving the desired contact force and collapsing of the heated electrode, have often resulted in disadvantages which have made projection welding less than desired for use on thin gauge steel sheets. For example, difficulties have often been observed with respect to marking or discoloration of the sheet surface, or if the sheets are coated, then damage to the coating often results. For these and other reasons, projection welding has previously not met with a high level of acceptance for use on thin steel sheets, although more recently a greater acceptance is being achieved.
In this regard, U.S. Pat. No. 4,417,122 as owned by the Assignee hereof relates to a desired projection welding arrangement wherein a low-inertia fast-response welding gun is provided for effecting contact with and projection welding between two overlapping light-gauge steel sheets. With the arrangement of the aforesaid patent, not only is there provided a welding gun having a low-inertia and hence fast response electrode, but more significantly the electrical current is applied to the electrode for only an extremely short time duration, which duration is only a fraction of one-half of a typical alternating current wave cycle. In this manner, a very high energy pulse of current, of very short time duration, is transmitted to the moving welding head and applied to the projection contact area between the overlapping sheets to thereby effect rapid and efficient welding of the sheets together. This system has been able to provide a desirable weld nugget between the overlapping sheets without causing any significant heat-effected zone, and without causing excessive discoloration or marking of the sheets. Hence, the arrangement of the '122 patent has in recent years become more accepted for projection welding, particularly for hem-type projection welding such as is conventionally utilized for securing thin gauge steel sheets as associated with automobiles and the like.
Referring to FIGS. 1-3, there is illustrated a known arrangement for effecting projection hem welding, which arrangement incorporates therein the projection welding arrangement of the '122 patent. This welding arrangement 10 is particularly desirable for projection welding of thin sheetlike steel components disposed in overlapping relation, typically light gauge steel sheets which commonly have a thickness in the range of 0.020 to 0.050 inch. The thin steel workpieces or sheets to be welded are illustrated at 11 and 12, which steel sheets have portions which directly overlap as shown in FIG. 1, and in a hem welding process one of the steel sheets 11 also has a flange or hem part 13 which is bent upwardly to overlap the remote side of the other steel sheet 12, whereby the sheet 12 is sandwiched between the sheet 11 and its hem part 13. The intermediate sheet 12 also is provided with a bead or projection 14 formed therein and projecting transversely therefrom so that the tip of the projection contacts the inner (i.e. lower) surface of the hem part 13. In this hem type arrangement, a projection weld is created directly between the sheet 12 and the hem part 13 at the region of the projection 14.
In this known projection welding arrangement 10 the overlapping steel sheets (i.e., the workpieces) are typically positioned on a backup support or die 15, and a movable welding head assembly 17 is positioned adjacent the die 15 so as to cooperate with the overlapping sheets to permit creation of the projection weld. The movable welding head assembly 17 includes a hollow housing or body 18 having an electrode 19 movably supported therein and projecting outwardly for contact with the overlapping sheets in the region of the projection, and a spring 21 (typically a coil spring) is confined within the housing and acts against an inner face of the electrode 19 so as to urge the electrode 19 outwardly into an extended position, in which position the electrode abuts an interior stop surface formed on the housing. The movable welding head assembly 17 is electrically connected to a stationary transformer 22. The power supply to the transformer 22, and hence the welding current supplied thereto to the welding head assembly, is in turn controlled by a suitable control unit 23. The transformer 22 has the primary coils 24 thereof connected to suitable electrical leads or conductors 25 and 26, which conductors typically have a conventional 460 volt, single phase, 60 hertz power supply connected thereto. The secondary coils 27 of the transformer are in turn connected to suitable conductors 28 and 29, one of which is connected to the welding head assembly 17 and the other of which is connected on a downstream side of the sandwiched workpieces, such as by being connected either to the die 15 or in a hem welding process being connected to an electrical contact gun 31 as illustrated in FIG. 1. This contact gun 31 includes a support 32 such as a conventional double-piston double-acting pressure cylinder, normally an air cylinder, having a conventional electrode 33 movably supported thereon, the latter being positioned for engagement with a part 34 of the intermediate sheet 12, which part 34 where it engages the electrode 33 being spaced from the overlapping or sandwiched portions of the sheets and also being spaced from the sheet 11. The conductors 28 and 29 are typically constructed as conventional flexible laminations so as to permit movements of the welding head assembly 17 and contact head assembly 31 relative to the transformer 22.
The welding arrangement also includes a drive device 36 for effecting movement of the welding head assembly 17. This drive device typically constitutes a pneumatic cylinder having a housing 37 which is typically stationary mounted, and provided with an extendible and contractible piston rod 38 which in turn couples to the housing 18 of the welding head assembly 17 to control the movement of the assembly 17 into engagement with the sheets when a projection welding operation is to be carried out.
In a typical projection welding operation, the drive 36 is energized and moves the head assembly 17 downwardly to cause the electrode 19 to engage the sheet part 13 adjacent or substantially in alignment with the projection 14, with the downward movement being sufficient to cause an inward depression of the electrode 19 and hence a compression of the spring 21. The control unit 23 then supplies a very short duration, unipolar, high energy electrical current pulse to the electrode 19 which effects heating of the projection 14 and of the sheet part 13 in the immediate vicinity of the projection. This, coupled with the pressure imposed on the electrode by the spring 21, effects collapsing of the heated projection to thereby create a weld bead or nugget 35 between the sheet parts 12 and 13 substantially as illustrated in FIG. 3.
When carrying out a hem welding operation, typically for welding of an outer automotive body sheet 11 to an inner support sheet 12, the overlapping edge portions of the steel sheets 11 and 12 typically have an adhesive 39 provided therebetween, but this adhesive is not effective for fixedly securing the sheets 11 and 12 together until subjected to high temperatures such as experienced in a paint drying oven. Accordingly, the projection weld is provided so as to at least temporarily fixedly join the sheets 11 and 12 together to maintain dimensional stability during subsequent handling and operations, and when the finished automotive part is ultimately painted and placed in the heat drying oven, then the adhesive 39 is activated to create a fixed securement of the sheets 11 and 12 together.
With respect to the creation of the projection weld, and referring to FIG. 4, in many of the known projection welding techniques the welding current applied to the electrode constitutes a low-frequency sinusoidal alternating-current wave form as shown at 41 in FIG. 4, and most typically several cycles of the current wave form are applied to the electrode to effect a single projection weld. In use of the welding head and projection welding arrangement disclosed in the '122 patent, and as depicted by FIG. 1, however, the electrode is subjected only to a very short duration, unipolar, high-intensity electrical current pulse 42 so as to effect a projection welding operation. Further, the control unit 23 preferably includes appropriate switching circuitry or switching software so that subsequent welding operations are of opposite polarity, for example the first weld being a unipolar pulse 42 as shown in FIG. 4, and the next or second weld being a similar pulse but of opposite polarity as indicated at 42'. The welding pulse which is used for each projection weld has a duration which occurs solely within a single half-cycle of the alternating-current wave form, and the duration of the current pulse such as pulse 42 is less than the time duration of the half wave form.
The specifics of the construction and operation of the welding arrangement 10 as summarized above is explained in the '122 patent, and further detailed description thereof is believed unnecessary.
While projection welding and particularly hem-type projection welding using a pulse-type welding head of the aforementioned type has proven fairly successful and is gaining in commercial acceptability for welding steel sheets, nevertheless it has been observed that such arrangement still has less than optimum characteristics. For example, with this type of welding head, the spring which biases the electrode is normally in either a non-compressed or minimally compressed condition when the electrode is fully extended and, consequently, due to the significant variation in the spring rate characteristics in springs of this type, it has been observed that each welding head when subjected to an inward electrode depression and spring compression of a predetermined amount nevertheless results in significantly varying spring force magnitudes, and this in turn adversely effects the repeatability and dependability of the projection welds being produced. Further, with this arrangement the electrode typically has to be depressed through a significant distance, often in the order of three-fourths inch, and accordingly causes a corresponding large cycling stroke or compression of the spring, which in turn decreases the life of the spring. This long stroke also has been observed to, in some instances, result in the electrode being extended outwardly by this spring so as to contact and damage the copper support plate when the metal sheets are not present thereon, and this in turn can require that the copper support plate be repaired or replaced. The variability of the compressed force of the spring when under a normal load or compression condition, prior to actual initiation of the heating and compressing operation so as to effect welding, also has been observed to adversely effect the quality of the weld nugget being formed since an optimum weld nugget is formed by controlling and coordinating the magnitude of the electrical current peak in conjunction with the compression force imposed on the electrode by the spring, all in turn determined in conjunction with the thickness of the metal sheets and the size of the projection, so as to result in optimization of the heating and compression and collapsing of the projection so as to create the desired weld nugget.
To improve on the projection welding arrangement as described above, the Assignee hereof developed an improved welding head which has demonstrated the ability to significantly improve on or at least partially overcome many of the inconveniences and irregularities discussed above. This improved welding head, as disclosed in Assignee's copending U.S.A. application Ser. No. 08/734,484 filed Oct. 21, 1996 (the disclosure of which in its entirety is incorporated herein by reference), has an electrode-biasing spring which is initially preloaded to always exert a significant biasing force on the electrode to maintain it against a stop and thus define a normal fully extended position of the electrode. The spring is initially precalibrated to exert an accurate predetermined biasing force on the electrode when the electrode is depressed inwardly and effects compression of the spring by a predetermined distance, which latter distance is normally of small magnitude to minimize spring wear and optimize the responsiveness of electrode movement. Thus, when the welding head is disposed in engagement with overlapping steel sheets in alignment with a projection, the electrode is depressed through said predetermined distance whereby a uniform predetermined biasing force is always applied to the electrode and hence to the projection weld area.
While the improved welding head of the aforesaid application has provided improved projection welding capability with respect to overlapping thin steel sheets, and has been achieving significant commercial acceptance, nevertheless the known projection welding process utilizing this improved welding head still is principally limited for use in projection welding of steel sheets.
Accordingly, while projection welding of ferrous or steel sheets has now reached a reasonable degree of development and commercial acceptance, nevertheless projection welding of thin nonferrous sheets such as aluminum is still generally considered not feasible, and in fact the welding of thin aluminum sheets continues to present difficulties or impose welding parameters which make welding thereof difficult and hence inefficient and expensive, and in many situations such is economically prohibited from a commercial standpoint.
When dealing with thin nonferrous sheets and specifically thin aluminum sheets, such as aluminum sheets in the range of from about 0.030 inch to about 0.050 inch, attempting to weld these aluminum sheets presents several technical problems due to the physical characteristics and properties of aluminum. A first problem with attempting to weld aluminum is the very low melting temperature thereof. The melting temperature of aluminum is normally in the range of about 1,100.degree. F. to 1,250.degree. F., this being in contrast to steel wherein the melting temperature is typically in the range of 2,400.degree. F. to 2,700.degree. F. A second and significant problem is that the plastic range of aluminum when subject to heat (that is, the temperature range below and up to the melting temperature in which aluminum can be readily forged or deformed while still in a solid state) is very narrow (in contrast to steel which has a fairly large temperature range defining the plastic zone). Control over the amount of heat supplied to thin aluminum sheet is thus precisely and accurately required in order to achieve sufficient heating to place and maintain the aluminum in a plastic condition, yet avoid unacceptable and undesired melting thereof. Thirdly, a further and significant problem experienced with attempts to weld thin aluminum sheets is the difficulty created by the oxide coating which inherently develops and coats the exterior surfaces of aluminum sheet upon manufacture thereof. The oxide coating on the exposed surfaces of aluminum sheet has a very high melting point, such as in the neighborhood of about 3,600.degree. F. If one attempts to first melt the oxide coating on a thin aluminum sheet, the high temperature required to do so is so high as to cause almost immediate melting of the thin underlying aluminum sheet and hence this effects spattering of liquid aluminum and effectively destroys the sheet in the area of the proposed weld. Lastly, the lower strength of aluminum (in contrast to steel) is believed to be such as to cause too rapid a collapse of a projection during projection welding, which rapid collapse prevents proper follow-up of the electrode, and hence prevents creation of a proper projection weld.
At the present time, welding of overlapping thin aluminum sheets is primarily carried out by spot welding, that is, a spot weld is created between two overlapping thin aluminum sheets which directly contact one another. This process is primarily utilized only in those industries where the precision and complexity associated with such process, and the significant cost thereof, is required. For example, spot welding of thin aluminum sheets is conventionally utilized in the manufacture of aircraft. Such spot welding process, however, requires that the aluminum surfaces be initially cleaned and treated to eliminate the surface oxide coating, which treating processes are complex and time consuming, and the spot welding operation must be carried out almost immediately thereafter to prevent reformation of oxide coating in the cleaned areas. In situations where the sheet aluminum is intended for spot welding, it is also conventional to apply a coating on the aluminum sheet at the forming mill, which coating minimizes the development of surface oxidation on the aluminum sheet, although this mill-applied coating must also be removed prior to use and spot welding of the aluminum sheets. Because of the extensive and necessary cleaning steps associated with spot welding of aluminum sheets, and the complications associated with the temperature properties thereof, spot welding of aluminum sheets is complex and costly, and often unreliable, and thus such process has been commercially adopted only where the product requirements do not permit substitution of more economical materials or processes.
Even in the aircraft industry, welding of thin aluminum sheets is primarily carried out by spot welding, and projection welding is not believed feasible or utilized, and in fact projection welding of aluminum sheets, as a general principle, has been generally considered not practically possible.
Another problem associated with spot welding of aluminum is that such technique typically requires significantly high amperage, such as up to about 40,000 amps, be applied to the overlapping sheets between the opposed electrodes, and such amperage is typically applied over a significant time period, such as up to seven full amperage cycles. For example, the spot welding of aluminum sheets is typically carried out by a process known as mid-frequency spot welding, the latter requiring a rather complex and expensive system. In this process the line current utilized is typically three-phase 440 volt AC, which is then converted to DC, and is then formed into a square wave current of high frequency or hertz, typically in the range of 800 to 1,500 hertz. This is then supplied through a controller to a transformer and is again rectified to DC and then supplied to the welding gun. Such mid-frequency welding arrangement, however, is very costly and also raises safety concerns inasmuch as very high voltage DC is supplied at a small transformer which is connected to and positioned close to the welding gun. Further, due to the high current intensity necessary to transfer the current from the curved contact faces of the electrodes into and through the contacting aluminum sheets, the tips of the electrodes tend to pick up a significant quantity of aluminum from the sheets during the welding thereof, and this significantly contaminates the electrode tips and accordingly greatly reduces the life thereof.
In other industries and specifically the automotive industry, overlapping steel sheets are often spot welded, and in recent years there has been an increased commercial acceptance and use of projection welding, as mentioned above. While there is a desire in the automobile industry to use increasing amounts of thin aluminum sheets so as to reduce vehicle weight, and while there has been attempts to utilize spot welding of thin aluminum sheets for automobile bodies, to the best of Assignee's knowledge such prior attempts have achieved only limited commercial acceptable since the requirements associated with the cleaning and treating of the sheets to effect removal of surface oxide prior to welding present sufficient manufacturing problems and costs as to make such spot-welding aluminum process generally unacceptable where mass production of products under economical cost procedures is required. At the present time, while there is a desire to utilize a greater amount of aluminum in the construction of automobiles, the joining of aluminum sheets continues to be a significant problem, and thus most aluminum sheets are currently joined either by mechanical fasteners such as rivets, or where welding is required, by mid-frequency spot welding.
While spot welding of overlapping aluminum sheets can be and is successfully performed, although normally only under complex and controlled processes as explained above, the welding of overlapping aluminum sheets by projecting welding, prior to the present invention, has generally been considered impractical due to the narrow plastic range of aluminum, the requirement of projection collapse during projection welding, and the many other complications which exists when attempting to weld aluminum sheets, such as explained above with respect to spot welding.
Accordingly, it is an object of this invention to provide an improved projection welding process and apparatus which is particularly desirable for permitting projection welding of nonferrous sheets, particularly thin aluminum sheets.
In the improved projection welding process and apparatus of this invention, one of the overlapping nonferrous (i.e. aluminum) sheets is provided with a projection formed therein, and the sheets are positioned so that the tip of the projection on one sheet contacts the surface of the other sheet. The sheets are not subjected to surface pretreatment, and the sheets typically have conventional surface oxide thereon when presented for projection welding. A projection welding head assembly, preferably the assembly of the aforementioned Ser. No. 08/734,484 application, is positioned with the electrode thereof engaged with one of the sheets, and the other sheet is typically positioned against a stationary backup or electrode. The electrode of the welding head then imposes, in rapid time succession, a series of welding pulses to the overlapping contacting sheets. The series of welding pulses includes two and preferably at least three pulses which progressively step up in amperage to provide for precise control of the heating of the projection and the associated contact area on the other sheet to cause heating to the plastic state, and then controlled forging (i.e. collapsing) of the projection occurs with subsequent and progressive creation of the desired weld nugget.
In a preferred embodiment of the improved process and apparatus, the series of weld pulses also preferably includes, at the end of the step-up pulses, one or two pulses which progressively step down in current magnitude so as to permit greater control over the plasticity of the projection and the forging together of the overlapping sheets to create the desired weld bead. With this arrangement, the contact pressure between the tip of the projection and the opposed sheet, and the step up in the electrical current transmitted therethrough, is believed sufficient to cause an initial breakdown of the oxide coatings at the contact zone without melting the underlying aluminum while enabling the initial creation of a small weld nugget at the contact zone. The progressive buildup of the current pulses then allows the current to pass around the edge of the nugget to permit the nugget to progressively enlarge. The current step-down pulses at the end of the cycle are provided to counteract the rapid cooling properties of aluminum, and hence this additional lower stage heating maintains the aluminum in the nugget area sufficiently heated to permit additional forging together of the two sheets due to the pressure applied thereto by the moving electrode.
In the improved projection welding process of this invention, as aforesaid, the electrode preferably initially imposes a squeezing or clamping force on the overlapping sheets which preferably causes a partial collapse of the projection, such as at the tip end thereof, to effect breaking of the surface oxide layer. Thereafter initiation of the series of welding pulses permits progressive heating and plasticizing of the projection and the progressive collapsing thereof and the progressive formation of a weld bead between the overlapping sheets.
The improved projection welding process and apparatus of the present invention has so far proven to provide unexpected results in that it has been at least experimentally proven capable of permitting projection welding of overlapping aluminum sheets without requiring pretreatment of the sheets to remove surface oxides, which such process has previously generally been considered impractical.
Other objects and purposes of the invention will be apparent to persons familiar with projection welding, and specifically the properties of aluminum, upon reading the following specification and inspecting the accompanying drawings.